In the production of liquid-crystal display, plasma display and the like, various treatments involving drug solution, such as resist solution, are performed. Drug solution is applied by first being pumped to a discharge nozzle using nitrogen gas and then by being discharged through the discharge nozzle. However, in the treatments involving drug solution, when the amount of dissolved gas in the drug solution is substantive, air bubbles can be generated due to the supersaturation of the dissolved gas. This is because the pressure being applied to the drug solution by the nitrogen gas is reduced to atmospheric pressure as the drug solution is discharged through the discharge nozzle. Intrusion of air bubbles into the drug solution, such as resist solution, causes treatment patches and creates problems such as patterning defect during the lithography step. Means for preventing treatment patches of drug solution include a method using spin coat in which the drug solution is degassed as it is being applied. However, with the recent trend for liquid-crystal displays and plasma displays to have ever-larger screens, it is increasingly becoming difficult to employ the method in which the drug solution is applied using spin coat.
On the other hand, a deaeration method using a hollow-fiber membrane as a deaeration membrane is employed for removing dissolved gas from drug solution and the like. In this method, membrane-mediated deaeration by the hollow-fiber membrane is carried out during the drug solution pumping step, in which the amount of dissolved gas in the drug solution is reduced and the generation of air bubbles is suppressed. As to the hollow-fiber membrane used in the membrane-mediated deaeration, composite hollow-fiber membrane having a non-porous homogeneous membrane layer with gas permeability and a porous membrane layer that supports the homogeneous membrane layer, is known. Examples of composite hollow-fiber membrane include the following.
(1) A composite hollow-fiber membrane having a three-layered structure wherein a gas permeable homogeneous membrane layer consisting of silicone-polycarbonate copolymer, silicone-urethane copolymer and the like, is sandwiched between two porous membrane layers consisting of polyolefin (see Patent Document 1, for example).
(2) A composite hollow-fiber membrane having a three-layered structure wherein a homogeneous membrane layer consisting of linear chain polyethylene is sandwiched between two porous membrane layers (see Patent Document 2).
(3) A composite hollow-fiber membrane having a porous membrane layer consisting of poly-4-methylpentene-1 and a homogeneous membrane layer (see Patent Document 3).    Patent Document 1: Japanese Unexamined Patent Application, Publication No. H5-185067    Patent Document 2: Japanese Unexamined Patent Application, Publication No. H11-47565    Patent Document 3: Japanese Unexamined Patent Application, Publication No. H6-335623